Use window.location.origin as default host in URLBuilder

	Closes #1695

ktor-http/common/src/io/ktor/http/URLBuilder.kt

@@ -24,7 +24,7 @@ const val DEFAULT_PORT: Int = 0
  */
 class URLBuilder(
     var protocol: URLProtocol = URLProtocol.HTTP,
-    var host: String = "localhost",
+    var host: String = originHost,
     var port: Int = DEFAULT_PORT,
     var user: String? = null,
     var password: String? = null,
@@ -84,6 +84,13 @@ class URLBuilder(
     companion object
 }
 
+/**
+ * Hostname of current origin.
+ *
+ * It uses "localhost" for all platforms except js.
+ */
+expect val URLBuilder.Companion.originHost: String
+
 /**
  * Create a copy of this builder. Modifications in a copy is not reflected in the original instance and vise-versa.
  */

ktor-http/js/src/io/ktor/http/URLBuilderJs.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2014-2020 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package io.ktor.http
+
+import io.ktor.util.*
+import kotlin.browser.*
+
+/**
+ * Hostname of current origin.
+ *
+ * It uses "localhost" for all platforms except js.
+ */
+actual val URLBuilder.Companion.originHost: String
+    get() = if (PlatformUtils.IS_BROWSER) window.location.origin else "localhost"

ktor-http/jvm/src/io/ktor/http/URLBuilderJvm.kt

@@ -12,3 +12,11 @@ import java.net.*
 operator fun Url.Companion.invoke(fullUrl: String): Url = URLBuilder().apply {
     takeFrom(URI(fullUrl))
 }.build()
+
+/**
+ * Hostname of current origin.
+ *
+ * It uses "localhost" for all platforms except js.
+ */
+actual val URLBuilder.Companion.originHost: String
+    get() = "localhost"

ktor-http/posix/src/io/ktor/http/URLBuilderPosix.kt

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2014-2020 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package io.ktor.http
+
+
+/**
+ * Hostname of current origin.
+ *
+ * It uses "localhost" for all platforms except js.
+ */
+actual val URLBuilder.Companion.originHost: String
+    get() = "localhost"